US 4,729,129 AGranted

VSWR protection circuit apparatus

AssigneeITT Avionics, A Division of ITT Corporation

InventorCharles B. Koerner

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Abstract

A VSWR protection circuit employs first and second mixers whereby the first mixer receives at one input the transmitted frequency and has a local oscillator coupled to the other input to provide at its output a difference frequency. This difference frequency is applied to the input of the second mixer which also receives at another input a frequency which is the sum of an undesired EMI signal plus a residue signal due to reflection from the transmitter. The second mixer produces difference frequencies according to the output from the first mixer. The output from the second mixer is applied to a narrow bandpass filter which essentially allows the transmitted frequency to propagate. The output from the narrow band filter is then detected whereby a DC control voltage is provided and used as a gain control for the transmitting amplifier and essentially is independent of most interfering frequencies.

Claims9

Dependent claims

A VSWR protection circuit for use in a transmitter which transmits a predetermined frequency to provide a gain control to a gain controlled transmitting amplifier having an output coupled to an antenna and to elements of the protection circuit, which antenna can undesirably respond to an EMI signal having a frequency close to the transmitted frequency comprising:

a source capable of providing an output frequency of a predetermined value,
first mixing means having one input adapted to receive said transmitted frequency and a second input coupled to said source to provide at an output a first mixed signal,
second mixing means having one input adapted to receive said EMI signal and a second input coupled to the output of said first mixing means to provide at an output a second mixed signal,
a narrow bandpass filter with an input adapted to receive said second mixed signal, said narrow bandpass filter having a center frequency centered about said output frequency of said source and having a bandwidth indicative of EMI signals close in frequency to said transmitted frequency,
detecting means having an input coupled to said bandpass filter to provide a DC voltage at an output indicative of the magnitude of said transmitted frequency and independent of said EMI signal which is applied as an input to said transmitting amplifier.

2 more elements

Dependencies are inferred from the claim text.